Car & Bikes says – Changes from BS3 to BS4 for motorcycles are more than just tailpipe emissions. Regulations also (for the first time) restrict evaporation emissions from fuel tanks, which mean a new breather value and additional or changed stamps in the body structure or additional brackets that hold these new systems. This is very difficult to retrofit considering the fact that major design changes might be needed.Coming to tail pipe emissions, BS4 motorcycles in most cases require larger catalytic converters in terms of flow volumes to eliminate harmful nitrogen based gasses. And although that could be adapted within the exhaust system, most bikes also need a secondary airflow system that adds oxygen (or natural air) to the exhaust stream just before the catalytic convertors. This of course, is a more complex system for which retro fitment is more difficult. Many manufacturers have also resorted to a more complex ECU and ignition control setup that has more than the standard idle map and full throttle map for a better control of the ignition timing which in turn makes for a cleaner burn and thus, less pollution. The byproduct of this is also a more linear acceleration curve and better fuel economy.